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Thinning the periphery of the top
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Author:  David Malicky [ Thu May 13, 2010 5:50 pm ]
Post subject:  Thinning the periphery of the top
Are there any common guidelines for thinning the perimeter of the top? For example, how much, and how far in? For my last GA, I thinned the perimeter of a 0.100" soundboard, tapering from 3" inboard to 0.075" at the edge. The guitar came out sounding nice--open, loud, with good thump, but bass-biased and weak on the high E string. (I can post a soundclip if helpful.) I know the treble weakness could be due to many factors; would any of you know if that amount of thinning is a likely contributor? I received some helpful feedback to not thin anymore on that one; I wondered if 0.075" (or -0.025") was pushing it. For my next build, I'm thinking of thinning in stages after stringing up, starting with -0.005 to -0.010" around the tail, then extend similarly to around the waist, then repeat, playing between each iteration.

I realize there are probably many approaches, depending on the tone goals. I'm looking for some guidance on techniques, what's typical, and what's too much, ... don't want to glue those pesky sawdust pieces back in place!

Thanks, David
Author:  woody b [ Thu May 13, 2010 6:03 pm ]
Post subject:  Re: Thinning the periphery of the top
There's alot of different ways to do things. I believe alot of people these days just leave the top thickness uniform. I would suggest working on other ways to voice until you think you've got a pretty good handle on it. Then start "graduateing" the top if you think it's needed. I thickness by deflection, but don't always use a uniform thickness, but don't thin all the way around the edges. I'll thin the lower bout some if I want more bass.
Author:  Laurent Brondel [ Thu May 13, 2010 7:34 pm ]
Post subject:  Re: Thinning the periphery of the top
I do thin the lower bout perimeter, perhaps not as drastically. But I usually start with a thicker top in any case. .100" seems pretty thin unless it's a 0 or smaller. Working the perimeter may add some bass, I know it does to some extent, but mostly I think it increases the dynamic range. I stop when I get the "right" amount of bounce tapping on the bridge area, and it changes slightly with every guitar. The place I do not recomend to thin out is the end block area BTW.
Author:  Darryl Young [ Thu May 13, 2010 8:02 pm ]
Post subject:  Re: Thinning the periphery of the top
Interesting Laurent. I would have guessed thinning above the tail block would help the bass most.
Author:  Laurent Brondel [ Fri May 14, 2010 10:24 am ]
Post subject:  Re: Thinning the periphery of the top
You want your top to be as thick as it can be over the tail block for structural reasons.
Author:  Burton LeGeyt [ Fri May 14, 2010 11:51 am ]
Post subject:  Re: Thinning the periphery of the top
Laurent,

I had not heard that. That is one area I do thin more than most. What is the consequence you expect from that? I have a hard time imagining what forces are acting there in excess compared to the rest of the rim edges.
Author:  Arnt Rian [ Fri May 14, 2010 12:11 pm ]
Post subject:  Re: Thinning the periphery of the top
On an instrument with a tailpiece it is common to leave some wood in this area, as the pull of the strings will try to buckle this part of the soundboard. This is often seen on arch top mandolins which are carved to thin here, for example. I too fail so see how this can be problematic on an instrument with a bridge that has the strings attached in the middle of the soundboard. I have not noticed any problems with the guitars that I have built this way (with soundboards thinned over the tail block area).
Author:  Laurent Brondel [ Fri May 14, 2010 12:48 pm ]
Post subject:  Re: Thinning the periphery of the top
Burton LeGeyt wrote:
I had not heard that. That is one area I do thin more than most. What is the consequence you expect from that?
It is something I learned at Pantheon. I think a couple of guitars there in the early days either had distortion over the neck block, or the glue joint came off. Of course it may be the odd cases among 1000s of guitars that would be fine, but I remain prudent. Besides the end block is where forces are concentrated in case of shock, dropped guitar etc.
Author:  Alan Carruth [ Fri May 14, 2010 1:00 pm ]
Post subject:  Re: Thinning the periphery of the top
I, too, have always felt that the tail block area was the safest place to thin the top. You can get 'dimples' there, of course, but most of the failures I've seen were from abuse, rather than normal loads.

What I've seen is that thinning between the bridge and the tail block tends to bring up the bass more, while thinning in the 'wings', between the end of the bridge and the wide part of the bout can help bring up the treble, or, at least, make the tone 'sharper' or more 'cutting'. I found myself thinning behind the bridge regularly, so now I do it from the inside, before I glue on the braces.
Author:  Darryl Young [ Fri May 14, 2010 9:17 pm ]
Post subject:  Re: Thinning the periphery of the top
Good information.......thanks for sharing,
Author:  David Malicky [ Sat May 15, 2010 6:56 pm ]
Post subject:  Re: Thinning the periphery of the top
Thanks very much, all, for the helpful replies. I'll look for the tone differences when I do the thinning selectively this time -- that will be interesting. And not go quite so far with it.
Author:  jfmckenna [ Sun May 16, 2010 7:07 am ]
Post subject:  Re: Thinning the periphery of the top
david82282 wrote:
The guitar came out sounding nice--open, loud, with good thump, but bass-biased and weak on the high E string.


And what would it had sounded like had you not done any thinning? ;)

I always thin out the tops on my classical guitars because I learned that and agree that it would seem to benefit the tone, so i just do it. I don't actually measure though but the numbers you posted seem reasonable to me, perhaps a little on the high side reaching 25% of the top thickness at the rim.

I've also felt that thinning at the tail block is not a good idea based solely on intuition. What I do is draw a wedge from the tips of the bridge down to the width of the tail block and leave that area alone.
Author:  Terence Kennedy [ Sun May 16, 2010 7:23 am ]
Post subject:  Re: Thinning the periphery of the top
I've been thinning the edges of the lower bout for several years after the box is closed but no binding. I hold the guitar against my chest and tap the top as I do it and I seem to be able to hear it "open up" at some point. I try to feel the bounce at the bridge that Dana describes but my confidence level is low there. Without the binding you can get some idea where you are with thickness and I seldom go below .090 except on a parlor I guess it helps, people seem to think they sound better and so do I so I'll keep doing it. I never thought about the tail block so I have thinned in that area too.
Author:  JJ Donohue [ Sun May 16, 2010 7:52 am ]
Post subject:  Re: Thinning the periphery of the top
I've always thinned as Terry has described.

Remember, if you thin the LB before the binding channels are cut, you get a more realistic tap sound to evaluate. Cutting the binding channels can break the air seal and change the tone completely until the bindings are installed. If you thin before the bindings are installed, you can then measure how much you've removed from the LB perimeter.
Author:  Terence Kennedy [ Sun May 16, 2010 8:42 am ]
Post subject:  Re: Thinning the periphery of the top
The binding brings up an interesting point JJ. If you thin the edges and then use a wider purfling like herringbone there isn't much top left actually glued to the lining. For this reason I only use a thin bwb purfling now or if I do use herringbone I glue it with LMI white. I've often wondered if a wide thicker purfling does not free up the edges of the top a little even if they were not thinned. I have no clue but maybe someone here does.
Author:  Ed Haney [ Sun May 16, 2010 8:55 am ]
Post subject:  Re: Thinning the periphery of the top
Alan Carruth wrote:
I, too, have always felt that the tail block area was the safest place to thin the top. You can get 'dimples' there, of course, but most of the failures I've seen were from abuse, rather than normal loads.

What I've seen is that thinning between the bridge and the tail block tends to bring up the bass more, while thinning in the 'wings', between the end of the bridge and the wide part of the bout can help bring up the treble, or, at least, make the tone 'sharper' or more 'cutting'. I found myself thinning behind the bridge regularly, so now I do it from the inside, before I glue on the braces.


What is the advantage of thinning on the inside before gluing the braces? That is, what are the pros and cons of thinning after the box is closed before routing the binding VS thinning the top before closing and even bracing the top?

Thanks for the teaching going on here.

Ed
Author:  Alan Carruth [ Sun May 16, 2010 2:13 pm ]
Post subject:  Re: Thinning the periphery of the top
Ed Haney asked:
"That is, what are the pros and cons of thinning after the box is closed before routing the binding VS thinning the top before closing and even bracing the top?"

I like to thin the top out in advance of gluing the bracing simply because it's easier and I'm lazy. Actually, with a forearm like mine, shaving braces afterward is difficult, especialy on classicals, so I've gravitated toward the Chladni method of tuning the top and back before I put the box together. The trick there is to keep good records and figure out what works and what doesn't, so you can do things early on in the process.

The flip side is, of course, that if you do make changes later, it's hard to know what the parts _should_ have been doing, as opposed to what they _did_. It's entirely plausible that any number of my guitars could have been better if I'd thinned the top out some way or other after getting them together, but then I would not have known how to duplicate the result. This way, if I hit it once, I know what I did and have a chance of hitting it again by doing the same thing.
Author:  Ed Haney [ Sun May 16, 2010 5:48 pm ]
Post subject:  Re: Thinning the periphery of the top
Thanks Alan.
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